sloproxy

feat(plugin): omit stream chunk history on payload chunks for schema v5 master

- Bump plugin schema version to 5 and introduce `SchemaVersionStreamChunkOmitHistory`. - Omit `HistoryChunks` on payload stream chunks for schema version 5+ to avoid per-chunk cloning and serialization overhead. - Conditionally accumulate and clone history chunks only when legacy plugins with schema version < 5 are active. Closes: #5451